Fabry-Perot (FP) filters have been widely used in tunable laser sources, including those with external cavities and ring cavities. In such lasers, the filter cavity is external to the laser cavity. The FP filter uses a classical wavelength tunable method, well known to one skilled in the art, so it is only briefly described here. A detailed description can be found in, for instance, K. K. Sharma, “OPTICS: Principles and Applications”, Academic Press; 1st edition (Aug. 30, 2006), which is incorporated herein by reference. The optical cavity of a tunable FP filter comprises two reflectors, a fixed reflector and a moveable reflector, separated by an air gap. Distributed Bragg reflector (DBR) mirrors, made from dielectric materials, are typically used as cavity reflectors because they provide high reflectivity.
The three critical parameters of an FP filter are its free spectral range (FSR), cavity finesse, and cavity filter bandwidth, but all of these depend on the effective optical cavity length and effective reflectivity. The effective optical cavity length of the FP filter needs to be used in order to more accurately calculate the filter parameters. This length includes the optical distance between the cavity reflectors and the optical penetration depth into the dielectric DBR mirrors. Optical distance is defined as the product of the physical (geometric) distance traveled by the light multiplied by the refractive index of the medium/media through which it propagates.
In a FP filter, the wavelengths of maximum transmission occur periodically, and the spacing between adjacent maxima (the mode spacing) is called the free spectral range, denoted by the symbol ΔλFSR. The FSR of a FP filter, for a given design wavelength λ, is determined by the effective optical distance between the cavity reflectors, Leff, i.e., the effective optical cavity length, expressed as:
                              (                                    Δ              ⁢                                                          ⁢                              λ                FSR                                      =                                          λ                2                                            2                ⁢                                  L                  eff                                                              )                .                            (        1        )            
In particular, this defining relationship shows that the FSR is inversely proportional to the optical cavity length: thus, a shorter cavity provides a broader FSR.
The finesse of a FP filter is determined by the effective reflectivity, reff, of the FP cavity, as follows:
      (                  F        FP            =                        π          ⁢                                    r              eff                                                1          -                      r            eff                                )    .The cavity reflectivity reff is, in turn, determined by the reflectivity of the two reflectors. The reflectivity lies in the range 0<reff<1.
The ratio of the FSR of a FP filter to its bandwidth is its finesse, FFP. This quantity is akin to a quality factor (“Q factor”) for the device. Therefore, narrowing the filter bandwidth for a constant FSR, leads to an increased finesse.
The filter bandwidth, δλFP, of a FP filter is the sharpness of each transmission peak. The bandwidth is determined by the reflectivity (and loss) of the interferometer plates as well as the plate spacing. It is also the ratio of the free spectral range (FSR) and the finesse (F), and is given by:
                              δλ          FP                =                                            Δλ                              FSR                ,                FP                                                    F              FP                                =                                                    λ                2                                            2                ⁢                                  L                  eff                                                      ⁢                                          1                -                                  r                  eff                                                            π                ⁢                                                      r                    eff                                                                                                          (        2        )            where λ is the design wavelength, and Leff is the effective optical cavity length of the FP cavity. The filter bandwidth is also its minimum resolvable bandwidth (i.e., its resolution). A narrow filter bandwidth is desirable for high optical transmission, low insertion loss, and low wavefront distortion. It can be seen that the key requirements to achieve a narrow filter bandwidth are: (1) a longer effective optical cavity length; and (2) higher reflectivity of the FP cavity reflectors.
A FP tunable filter can be tuned to selected wavelengths by changing the filter cavity length. The wavelength tuning range, Δλ, of a FP filter is given by:
                    Δλ        =                  λ          ⁢                                    Δ              ⁢                                                          ⁢              L                                      L              eff                                                          (        3        )            where ΔL is the change in cavity length of the FP cavity. The wavelength tuning range is typically smaller than the FSR for the FP filter. To achieve a wide wavelength tuning range (for example of 100 nm or more), the effective optical cavity length should be in the range of a few microns.
Micro-electro-mechanical-system (MEMS) technology has been used to change the filter cavity length (and hence the effective optical cavity length) of an FP filter to enable wavelength tuning. For example, one such tunable Fabry-Perot filter that uses a MEMS is disclosed in U.S. Pat. No. 6,373,632 B1, issued Apr. 16, 2002 to Flanders for “Tunable Fabry-Perot Filter”, which is herein incorporated by reference.
In this patent, two reflectors that define the FP cavity are situated on two separate wafers that are then bonded together. Ensuring that the mirrors are parallel to one another after this process is important. Motion comes from moving a membrane on one side of the FP cavity. There is one cavity for the filter; and another adjacent cavity defined by the MEMS. Although bonding two chips gives flexibility to configure the filter optical cavity length for a desired bandwidth and FSR, bonding two chips adds to the fabrication complexity and manufacturing cost.